The invention relates to compositions for implantation into the human and animal body.
These implantable compositions serve in particular for the replacement or repair of bones or teeth as well as parts thereof. They are particularly preferably used in the field of dentistry, i.e. in teeth and the bones of the masticatory apparatus, and therefore as dental materials. The compositions consist in particular of powder mixtures and admixing liquids, which are mixed directly before use, i.e. implantation, to form curing pastes. An essential part of the powder mixture comprises a powder of differing composition containing calcium phosphate. The admixing liquid is e.g. demineralized water or an aqueous solution, with the addition of substances, inter alia for improving the biological degradability and/or the acceleration of the tissue integration. Antibiotics and/or disinfectants can also be added to the compositions, to avoid infections.
The aim of the present invention is to avoid disadvantages which are present in the implantation systems for bone replacement corresponding to the state of the art. These disadvantages are as follows:
The following is disadvantageous when using bone implants made from solid material:
A costly adaptation involving the implant or the bone tissue is necessary in order that the hole to be filled in the bone tissue is completely closed.
When using implants which can be admixed to produce pastes, the previously mentioned disadvantage does not occur, but there are a series of other disadvantages, as follows:
The curing times of the admixed pastes are sometimes very short, which is a major hindrance during complicated operations. There are also pastes with very long curing times, which is also undesired.
After they are introduced into the bone tissue and before curing, the pastes are in part not resistant to liquids such as e.g. blood, i.e. they absorb liquid and soften.
The strength values of the cured pastes are sometimes low. In particular, the initial strength is in many cases too low.
In the paste-based implant materials which have become known, the rate at which the tissue integration and resorption occurs is only low.
Such implants are either not at all, or only slightly, biodegradable.
Details on this point emerge from the state of the art as described below:
The mineral bone material occurring in human and animal bodies consists predominantly of a hydroxyl apatite-like structure, the essentially occurring elements being calcium and phosphorous. Other elements also occur, however, in the bone substance, such as sodium, calcium, magnesium and barium.
The replacement of lost bone tissue by natural bone still presents considerable problems even today. This applies in particular in maxillofacial surgery, in accident surgery as well as in orthopaedics. These problems result from the limited availability of autologous bone or the great cost of procuring it, or the insufficient quality when using allogenic bone, combined with the risk of the transfer of diseases.
However, the synthetic bone replacement materials such as Bio-Oss(copyright), Synthacer(copyright) or the bioglasses, which are available at present as a solid substance also have considerable disadvantages. These are e.g. that due to their preformed, granular structure, they are not to be optimally implanted into the osseous defects. This applies in particular in dentistry because of the lack of space in the oral cavity, where preformed bone replacement is to be positioned only with great difficulty. A further disadvantage of these sometimes sintered materials is that although they are integrated as bone, they are not completely degraded and replaced by natural bone.
For some time, calcium phosphate-containing powders have also been used, which are present initially as a paste after being admixed with water or aqueous solutions and then cure after a certain time. After curing, these materials display a structural similarity to the mineral phase of natural bone in the x-ray diffractogram. There are thus no longer problems with the adaptation to bone defects of the patient, as the pastes exactly match the existing structures.
Disadvantages occur, with the paste-based implants which have become known according to the state of the art, in the curing times, resistance to liquids acting from the outside, strength after curing, tissue integration as well as biodegradability. These matters are discussed below, in the further description of the state of the art.
One of the first powder- or paste-based implants is the BoneSource(copyright) invented by Brown and Chow in 1965, in which the powder component consists of CaHPO4 and tetracalcium phosphate. After admixing with water, a curing cement paste results, the cured end-product of which is a nano-crystalline hydroxyl apatite with a calcium/phosphorous ratio (Ca/P) of 1.67 (i.e. 1.67 calcium atoms per phosphorous atom). A disadvantage of such a calcium phosphate cement is its relatively long curing period and the disadvantageous property of decomposing in the early phase of setting when in direct contact with liquids. This applies in particular in the case of direct introduction into bleeding bone wounds.
In the publication by Driessens et al, Bioceramics Vol 9, 1996, pages 231-234, amorphous calcium phosphate cements are described, the calcium/phosphorous ratio of which (Ca/P) is less than 1. These cements consist of Ca(H2PO4)2.H2O=mono-calcium phosphate monohydrate (MCPM) and CaKPO4 or mixtures of MCPM and Ca2NaK(PO4)2. As a result of their relatively high solubility in body fluids, they have lower compressive strengths than the previously described cements with a high Ca/P ratio. The reduced compressive strength is a disadvantage of these implant materials.
On the other hand, a degree, albeit small, of solubility of such materials in the body fluids is desired, as the materials can then be opened up more quickly by body cells, compared with cements which are insoluble in human or in animal body fluids and must essentially be described as osteotransductive.
The system comprising MCPM and CaKPO4, apart from the disadvantage of its low compressive strength after curing, also has the disadvantage that its working time is less than 3 minutes.
This working time is clearly to short for difficult implant operations. The MCPM and Ca2NaK(PO4)2 system also mentioned has higher compressive strengths, but the curing period is clearly too long compared with the system comprising MCPM and CaKPO4.
Such cement systems and the products which result from them after curing, such as Ca(H2PO4)2.H2O (=MCPM), hydroxyl apatite Ca10(PO4)8(OH)2 and others are known from patents U.S. Pat. Nos. 4,673,355, 5,053,212 or EP 0543765. The resorbability of these materials as well as the curing period are, as mentioned, still not satisfactory however.
For this reason, the object of the invention is to develop new implantable compositions with which curable, calcium phosphate-containing pastes can be admixed. Improved biological properties were important for the clinical success in patients, but also more favourable handling properties for the doctor providing treatment e.g. as regards the curing time. The composition according to the invention fulfils these conditions in an ideal manner.
It offers in particular the following advantages:
As it can be admixed to produce curing pastes, the result is an ideal adaptability to the existing bone structure in the patient.
Through variation of the composition, the curing time can be changed, which is extraordinarily advantageous during use.
During curing, the composition is resistant to liquids acting from outside, such as e.g. blood, which can occur with bleeding bone wounds.
After the implant paste has cured, a high initial strength of the implant very quickly results.
The tissue integration can be considerably accelerated by additives.
The special type of composition leads to a good biological degradability, the speed of which can be influenced by the added additives.
In the case of the implants corresponding to the state of the art, these positive properties are not present in combination with each other, so that the composition according to the invention represents a considerable technical advance.
The implantable composition according to the invention is characterized in that it contains a base mixture of
(a) CaKPO4 
(b) Ca2NaK(PO4)2 and
(c) Ca(H2PO4)2.H2O
The combination of (a), (b) and (c) referred to as base mixture is present in particular in the form of a powder mixture.
The subject of the present invention is furthermore the combination of suitable mixtures of calcium- and phosphorous-containing powders with different powdery additives and admixing liquids which consist of water or aqueous solutions with various additives.
The main feature of the calcium- and phosphorous-containing powder mixtures according to the invention is that they contain as essential component special mixtures of CaKPO4, Ca2NaK(PO4)2 and Ca(H2PO4)2.H2O (=MCPM) which are designated base mixtures. Further calcium phosphate-containing powders are preferably added to these base mixtures. The admixing of other substances is however also possible. The afore mentioned disadvantages with the cement systems corresponding to the state of the art are avoided with the compositions according to the invention.
A preferred powder mixture of the composition according to the invention consists of the base mixture with addition of CaHPO4.H2O and/or CaCO3, further variable proportions of other calcium phosphate-containing substances and other additives being able to be added. By varying the quantity ratios of the individually named components, the properties of the composition according to the invention can be changed over a wide range and thus be adapted to the respective need.
Furthermore, the composition according to the invention preferably contains an admixing liquid which can in particular contain water. With this liquid the composition can be processed to a paste which is applied to the substrate to be treated, moulded and finally cured.
The admixing liquids used for admixing and curing can, as mentioned, contain additives with which the positive properties of the composition can also be decisively influenced. This applies e.g. as regards the biological accessibility of the cured cement for the body""s own cells, as the tissue integration depends quite essentially on this.
The admixing liquid contains an aqueous solution of mannose-6-phosphate (M-6-P) in a preferred formulation. In the case of some formulations, it is also provided that saccharose-octasulphate is added to the admixing liquid. Mixtures of mannose-6-phosphate and saccharose-octasulphate in the admixing liquid are also provided for. The two named substances can however also be added to the powder component in certain formulations. In a preferred formulation, the saccharose-octasulphate is used as sodium or potassium complex salt.
It is also possible that antibiotics and disinfectants are added to the admixing liquid so that infections do not occur as long as the implant is not yet integrated into the tissue. Through the implantation of initially still unactivated calcium phosphate cement, there is always the risk of colonization by germs, as the cement is not yet resorbed and/or opened up in cellular or vascular manner. There is therefore a justified demand on the part of the user to provide such materials with substances which inhibit colonization by germs, or, by release of germ-inhibiting orxe2x80x94destroying active ingredients into the surroundings of the implant, protect the latter from the colonization by germs, or to treat the surroundings by releasing medicaments along the lines of a drug delivery system or active ingredient reservoir.
This is achieved by adding antibiotics or other germ-inhibiting and/orxe2x80x94destroying substances such as e.g. biguanide (e.g. chlorohexidine) to the cement powder. The antibiotics and/or disinfectants can however also be added to the admixing liquid which is used for curing the cement powder.
It is also possible to use as admixing liquids antibiotics-containing or disinfecting aqueous solutions such as are also commercially available. In this context, the following substances are preferably used:
Gentamicin containing injection solutions such as e.g. Duragentamicin 80 and Duragentamicin 160 (from Durachemie), tobramycin solutions or clindamycin-containing solutions such as e.g. Sobelin Solubile 300, 600 and 900 mg, or metronidazole-containing solutions such as e.g. Clont i.v. (from Bayer) or also metronidazole i.v. (from Braun). A Lavasept solution (from Fresenius) prepared with NaCl is also suitable as a disinfecting solution.
The composition according to the invention has the advantage that the curing speed and the compressive strength of the cured composition can be influenced in the desired way in particular by varying the quantity ratios of CaKPO4 and Ca2NaK(PO4)2 together with MCPM and, if present, CaHPO4.H2O. High proportions of CaKPO4 and MCPM result in short curing times with a somewhat lower compressive strength. Conversely, a greater compressive strength can be achieved by higher proportions of Ca2NaK(PO4)2 and MCPM, somewhat longer curing times then resulting.
Further possibilities for influencing the system properties result from the addition of variable proportions of CaCO3 and/or nanoparticular apatite (corresponding to EP 0 664 133 A1). Higher proportions of nanoparticular hydroxyl apatite give the desired good biological degradability of the cured composition in the patients"" body. Higher proportions of CaCO3 lead to a high strength after curing, the curing time being somewhat lengthened.
Further, it was shown that the curing speed of the powder/liquid mixture clearly increases through the addition of aqueous Na2HPO4 solution with concentrations of in particular up to 5%, so that the operator has the possibility of selecting a curing speed which is optimal for the present case.
It emerges from the publication by Martinez et al, J-Cell-Biochem, 59(2), 1995 that osteoblasts have a receptor for mannose-6-phosphate (M-6-P), the formation of bone by osteoblasts being stimulated by intracellular signal transmission. An improved fracture healing in rabbits through the administration of saccharose octasulphate was reported by Young et al., Invest. Radiol. 26(5), 1991. The saccharose octasulphate is said to act via the binding of locally present growth factors such as EGF and FGF (Szabo et al., Scand-J-Gastroenterol, 185, 1991) and/or stimulation of the endogenous prostaglandin system (Stern et al, Am-J-Med, 83(3B), 1987). The positive effect of the saccharose octasulphate on bone growth was however not yet mentioned previously in the literature in connection with reaction systems for bone replacement. To this extent, the addition of this substance to the composition according to the invention is a preferred version.
A further important feature of the present invention relates to the combination of M-6-P and saccharose octasulphate as additives for the base mixture (CaKPO4, Ca2NaK(PO4)2, MCPM) in combination with CaHPO4.H2O and CaCO3. The saccharose octasulphate can be introduced into the cement in the form of an aqueous solution as a liquid component or added in powder form to the mineral powder component. In this way, the object of preparing a calcium phosphate cement with the mentioned user-optimized curing properties and good strength after curing, which at the same time has the ability to bind growth factors, is achieved.
A cellular opening up of the implant via the formation of new blood vessels is thereby made possible. In addition, osteoblasts for the formation of new bones are also stimulated.
Through this combination, an intensified biological activity and thus also a resorbability of the synthetic bone replacement material is achieved. Resistance to liquids such as e.g. blood results from the cohesion of the components used.
Mixtures with the following relative weight proportions have proved to be particularly suitable formulations for the base mixture (CaKPO4, Ca2NaK (PO4)2 and MCPM):
The proportion of CaKPO4 and Ca2NaK(PO4)2 in the overall base mixture lies between 80% and 65%.
The proportion of MCPM is accordingly 20% to 35%
The ratio of CaKPO4 to Ca2NaK(PO4)2, relative to the overall quantity of these two substances, can vary within wide limits. Thus the proportion of CaKPO4 can lie between 1% and 99%, the proportion of Ca2NaK(PO4)2 accordingly lying between 99% and 1% .
It is preferable to use the three substances of the base mixture in an approximate molar ratio of 2 mol CaKPO4, 1 mol Ca2NaK(PO4)2 and 1 mol MCPM.
The preparation of the two components CaKPO4 and Ca2NaK(PO4)2 used for the reaction system according to the invention is carried out in particular as described hereafter.
Firstly, 1 mol K2CO3 is mixed with 2 mols CaHPO4 and the powder mixture is calcined for approx. 1 hour at 1000xc2x0 C. A rapid cooling to temperatures  less than 700xc2x0 C. follows the calcination. The cooling period must not be greater than 1 minute.
The Ca2NaK(PO4)2 is prepared analogously to the CaKPO4, i.e. the same process parameters are used for calcination and cooling. The powder mixture before calcination consists of 1 mol K2CO3, 1 mol Na2CO3 and 4 mols CaHPO4.
It is preferable that the components CaKPO4 and Ca2NaK(PO4)2 contained in the composition according to the invention are calcined at 1000xc2x0 C. and then brought by rapid cooling to less than 700xc2x0 C. within 1 minute.
To further improve the biological properties of the composition according to the invention, biologically active proteins such as growth factors can be added both to the powder mixtures and to the admixing liquids. Fibroblast growth factor (FGF), bone morphogenetic proteins (BMPs), enamel amelogenins or elastase inhibitors such as e.g. AEBSF (P 2714 from Sigma) can be considered for example. Bone morphogenetic proteins are substances which influence the protein metabolism upon the synthesis of new tissue in the direction of a particular structure class, in this case in the direction of bone growth. Enamel amelogenins are likewise bone morphogenetic proteins which are responsible in particular for the growth of the periodontal apparatus.
The compositions according to the invention comprising powder mixtures and admixing liquids produce implantation pastes which set at both room and body temperature.
The cured implantation pastes have compressive strengths which depend on their composition and can be more than 5 MPa. They are characterized by a curing time which can be set by the user within wide limits. A preferred embodiment is the use of a powder mixture according to example 2, by which the development of the material, initially inanimate in cellular terms, is very positively influenced by cells of the human or animal body.
The invention relates, finally, to the use of the composition to repair or replace bones, teeth or parts thereof. In particular, the procedure is such that the composition is applied to the selected substrate in the form of a paste, moulded and cured.
Various formulations have proved to be particularly advantageous for the different applications. In the following, some of the particularly advantageous formulations are described in examples 1 to 10. However, other formulations are also provided for.